The developing frog (e.g., clawed frog, Xenopus laevis) offers unique opportunities to attack fundamental immunological problems that are not restricted by the phylogenetic position of the experimental animal in which they are studied. Research is proposed with amphibians that addresses three such areas of developmental interest. To approach the first - the embryonic origins of T and B lymphocytes - chimeras will be constructed between embryos of different chromosomal complements and the derivatives of each component will be analyzed by flow cytometry. Sites of lymphopoiesis in larvae will also be examined by immunofluorescence. Finally, if a thymus-controlled alternative pathway of lymphopoiesis can be confirmed, its regulation will be examined as will its role in contributing stem cells to larvally thymectomized frogs that differentiate along a pre-T cell pathway. To approach the second area - thymic education of MHC restricted T-cells, chimeras will be created during embryonic life such that all lymphocytes will be of one MHC haplotype and the thymus of another. MHC restriction of T cells from such chimeras that never passed through their own thymus will be evaluated in cell cooperation studies. The third area addresses the issue of the ontogeny of self and nonself tolerance during metamorphosis. Adoptive transfer and in vitro techniques will be applied to analysis of the mechanisms of tolerance and of a disease that may reflect the failure to achieve such self tolerance.